It is well known that the purity of diphenols is of paramount importance regarding the quality of polymers which are prepared therefrom. Isomeric diphenols and other impurities specified herein that often accompany the desired p,p'-diphenols are often deleterious since such impurities do not participate as well in and may impede the polymerization processes. It is thus desirable and important to obtain the p,p'-diphenols in their highest purity in order to secure the quality of the polymers which are prepared therefrom. Since isomeric diphenols always accompany the desired p,p'-diphenols, purification of the crude reaction products is always necessary.
Purification of crude mixtures of p,p'-diphenols is often effected by the use of organic solvents such as benzene, methylene chloride or toluene. The use of these and similar solvents results in costly purification due to both the cost of solvents and the relative inefficiency of the method. Another purification method is set forth in U.S. Pat. No. 3,919,330. In this method crude 2,2-bis(4-hydroxyphenyl)propane is dissolved in ethylene glycol and the 2,2-bis(4-hydroxyphenyl)propane is recovered after partial precipitation with water. However, this method requires the use of large amounts of ethylene glycol and the necessity of an additional step of recovering the anhydrous glycol from the aqueous mother liquor in order to reuse it. Furthermore, although this process teaches a method of separating the desired 2,2-bis-(4-hydroxyphenyl)propane from its more soluble isomer, it does not effect removal of the less soluble impurities from the crude product.